Eternal Darkness: Sanity's Requiem
At the end of the game, you can choose one of three artifacts. Beat the game three times and keep choosing a different artifact. When you beat the game for the third time, you'll get a big special ending.
Beat game once and save clear data to unlock the ''Credits'' option. Carry over the saved data to a new game and beat the game a second time (select a different artifact) to unlock ''Jump to Game'' option. Beat the game a third time to unlock ''Eternal Mode." Eternal Mode is accessible from the Jump to Game option only.
When you play as Dr. Edwin Lindsey (the archaeologist), you destroy a field blocking one of those "small dog" holes, which you find out later is a clue to the use of the Summon Trapper spell. Usually, you will pass this area and continue on, since the spell you need isn't available - not only do you not have the scroll but a rune is missing. A little bit later you will acquire the necessary rune (TIER) to complete the spell.
When you have the TIER rune, head back towards the hole and send a trapper through. You will find two horrors inside, along with a pad displaying the purple Manotorok symbol and a spider web. Placing the trapper on the pad will open a door opposite the hole, revealing the Mantorok rune. A horror will then break open the wall opening the path to the spider web which hides the Mantorok codex.
The Mantorok rune is strong against all three others, and poisons the enemy when used as a shield or enchanted weapon. It dispels all types of damage fields, although the opposing rune is still needed for Reveal Invisible. Enjoy!
Thanks to Revolution readers Tyler Froehle, DevilFyre, Apollo and Noah!